Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, or other like materials.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
A dye is a colorant which is molecularly dispersed or solvated by a carrier. The carrier can be a liquid or a solid at room temperature. A commonly used carrier is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness.
Pigment based inks have been gaining in popularity as a way of addressing these limitations. In pigment-based inks, the colorant exists as discrete particles. These pigment particles are typically treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. Pigment-based inks suffer from a different set of deficiencies than dye-based inks. These deficiencies are the result of the fact that prior-art pigment-based inks display particle sizes and particle size distributions which can vary widely from pigment to pigment. This often requires costly and time-consuming filtration steps and often leads to plugging of the orifices of the ink jet printer. This dramatically limits the number of available pigments which can be used in ink jet printing processes. For example, although pigments in a wide variety of colors have been described in the prior art, commercially available ink jet inks have thus far been limited to black inks based on carbon black.
U.S. Pat. No. 5,679,138 describes a process for milling a pigment for use in an inkjet ink where the pigment is milled using rigid milling media until 90% by weight of the pigment particles have a size less than 100 nm. The rigid milling media are preferably polymeric milling media having an average diameter of 50 microns which is considerably smaller than conventional ceramic milling media. Beads used in the example are highly crosslinked polystyrene such as poly(styrene-co-divinylbenzene) 20/80.
U.S. Pat. No. 5,262,269 teaches a method of making pigmented toner particles produced by a limited coalescence technique resulting in particles of a non-crosslinked solid polymer matrix with pigment dispersed in the interior regions of the polymer matrix. The pigment can be a magnetic oxide, including ferric and ferrous oxides, and cobalt oxides.
There is a need for polymeric milling media particles with high density in order to improve milling efficiency by reducing milling time or by decreasing the ratio of milling media to un-milled pigment dispersion required per batch. There is also a need to reduce handling and cleaning difficulties in manufacturing during the use of small rigid polymeric milling media.